1. Field of the Invention
The present invention relates to liquid crystal display using ferroelectric liquid crystal and a manufacturing method of the same.
2. Description of the Related Art
Since liquid crystal display has features that it is thin and is low in power consumption and other features, the use has been expanding in various articles from large-sized displays to portable information terminals and the development has been actively made. Conventionally, for liquid crystal display, a TN system, an STN multiplex driving system, an active matrix driving system in which thin film transistors (hereinafter, it may be referred to as “TFT element”) are used in TN, and others have been developed and made practicable. However, nematic liquid crystal is used therein; therefore, the response speed of the liquid crystal material is as small as several milliseconds to several tens of milliseconds and it cannot be said that these sufficiently cope with display of moving images.
On the other hand, ferroelectric liquid crystal exhibits a very short response time in order of microseconds, thus ferroelectric liquid crystal is suitable for high-speed devices. As such ferroelectric liquid crystal, there is well known a bistable liquid crystal suggested by Clark and Lagerwall and which has two stable states when no voltage is applied thereto. However, the liquid crystal has a problem that the liquid crystal has memory property but graduation display cannot be attained since the switching thereof is limited to two states, namely, bright and dark states.
In recent years, attention has been paid to ferroelectric liquid crystal, in which the liquid crystal layer thereof is stable in a single state (hereinafter referred to as “monostable”) when no voltage is applied thereto, as a liquid crystal which makes it possible to attain graduation display by the matter that the director (the inclination of the molecule axis) of the liquid crystal is continuously changed by a change in applied voltage so as to analogue-modulate the light transmission thereof (NONAKA, T., LI, J., OGAWA, A., HORNUNG, B., SCHMIDT, W., WINGEN, R., and DUBAL, H., 1999, Liq. Cryst., 26, 1599).
As a method for having the mono-stability of the ferroelectric liquid crystal, there is a polymer stabilizing method for adding an ultraviolet curable monomer in the liquid crystal material, injecting into a cell, and curing so as to stabilize the alignment of the liquid crystal is known. Moreover, a method of obtaining the mono-stability of the ferroelectric liquid crystal by heating a ferroelectric liquid crystal not having the smectic A phase in the phase sequence so as to have the phase transition directly from the nematic phase to the chiral smectic C phase via the cholesteric phase in the temperature cooling process, to a temperature higher than the phase transition point of the cholesteric phase-nematic phase, and cooling down the same gradually, is known.
However, according to the polymer stabilizing method, problems are involved in that the process is complicated, the driving voltage is high, or the like. Moreover, according to the latter method not using the polymer stabilizing method, two different domains in the layer normal direction (hereinafter it may also be referred so as the “double domains”) can easily be formed so as to give rise to the grave problem of the display with the black and white inverted at the time of the drive. As to the double domains, although a method for having the mono domain by the electric field induced technique (, which uses DC voltage during cooling process) of gradually cooling while applying the voltage, is known (PATEL, J., and GOODBY, J. W., 1986, J. Appl. Phys., 59, 2355.), there is a problem of the alignment disturbance in the case the ferroelectric liquid crystal again has a temperature higher than the phase transition point, and thus the practical usability is low.
Moreover, as a technique for align the liquid crystal, there also are the rubbing process, the photo alignment method, or the like. According to the rubbing process, which provides the alignment ability by rubbing the polyimide-coated surface, it is difficult to have the evenness at the time of processing a large area, and moreover, there are the problems of generating the static electricity, the dusts, or the like. On the other hand, according to the photo alignment method, which provides the alignment ability by arranging the molecules in a specific direction by irradiating a compound having the photo alignment property with an ultraviolet ray, or the like, it is advantageous in that the quantitative alignment process can be controlled without generation of the static electricity or the dusts, however, a problem is involved in that the device cost is required due to the need of the exposing process.